A progressive-power lens is widely used in general because of an advantage that in appearance it is hardly detected from others as a spectacle for the aged in spite of a spectacle lens for presbyopia, an advantage that it allows a wearer to clearly look continuously from a far distance to a near distance without discontinuity, and so on. However, due to the necessity of arrangement of a plurality of visual fields such as a field for looking far, a field for looking near, and a field for looking at a distance intermediate therebetween, without a boundary line existing within a limited lens area, each visual field is not always sufficiently wide. Moreover, it is widely known that it presents disadvantages specific to the progressive-power lens such that there is a region mainly in a side visual field which causes the wearer to feel distortion or sway of an image.
Various proposals have been made since long ago to improve the disadvantages specific to the progressive-power lens, and most of such conventional progressive-power lenses have a surface structure created by a combination of a “progressive surface” arranged on an object side surface and a “spherical surface” or an “astigmatism surface” arranged on an eyeball side surface. Conversely to those, Atoral Variplas as a progressive-power lens, which is characterized in that a “progressive action” is added to the eyeball side surface, is released in 1970 from Essel Optical Co. (now Essilor), France.
Besides, recently proposed prior arts include, for example, technologies described in Patent Document 1 and Patent Document 2 and so on, which are generally called rear surface progression (or concave surface progression). The surface structure in the rear surface progression proposed in the Patent Document 1 has a main purpose of improving distortion and sway of an image by allotting a portion or the whole of a necessary addition diopter from an object side surface to an eyeball side surface to reduce the magnification difference of an image between a distance portion and a near portion.
Specifically, in the Patent Document 1, the object side surface is made as a spherical surface or a rotationally symmetrical aspherical surface to completely offset the “progressive action,” and a “progressive surface” providing a predetermined addition diopter is added (fused) to only the eyeball side surface. Besides, the Patent Document 2 proposes that the addition diopter on the “progressive surface” being the object side surface is made lower than a predetermined value and a “progressive surface” providing a deficiency in addition diopter is added (fused) to a “spherical surface” or “astigmatism surface” on the rear surface side.
Although having different purposes and reasons, other prior arts of the progressive-power lens having a structure of adding the “progressive action” to the eyeball side surface include, for example, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, and so on, and moreover, prior arts in which the “progressive action” is provided to both surfaces of a lens as in one described in the Patent Document 2 include, for example, Patent Document 7 and Patent Document 8. Commonly, in these prior arts, a combination of two surfaces of the object side and the eyeball side of a lens gives a necessary addition diopter.
[Patent Document 1] WO97/19382
[Patent Document 2] WO97/19383
[Patent Document 3] Japanese Patent Publication No. Sho 47-23943
[Patent Document 4] Japanese Patent Application Laid-open No. Sho 57-10112
[Patent Document 5] Japanese Patent Application Laid-open No. Hei 10-206805
[Patent Document 6] Japanese Patent Application Laid-open No. 2000-21846
[Patent Document 7] Japanese Patent Application Laid-open No. 2000-338452
[Patent Document 8] Japanese Patent Application Laid-open No. Hei 6-118353